NOTE: the nosotros form for -ar verbs and -ir verbs looks exactly like the Present Indicative Tense. (the first tense you learned) It is the -er verb that is easy to distinguish between the Present and the Preterit. To form the Preterit, drop the infinitive ending (-ar, -er, -ir) and add the endings.

THE PRETERIT OF IRREGULAR VERBS:

The verbs listed below are irregular in the Preterit. (Note the stems to which you add the endings.) They ALL have the following endings (whether they are -ar, -er or -ir verbs) WITH NO ACCENT MARKS.
endings: -e, -iste, -o, -imos, -isteis, -ieron (OR -eron if the stem has: j, ll or ñ.

The following irregular verbs have an ACCENTED “i” except in the third person (singular and plural) where “i” changes to “y.” It has to do with a dipthong and “u” and “i” are WEAK. (say that one outloud and it makes more sense?)

The verbs dar, ser and ir are also irregular in the Preterit. Dar takes the endings of regular -er, -ir verbs; ser and ir have the same identical forms in the Preterit. Look for an “a” that you will find when the verb is “ir” to distinguish them.

And, NO, we have not yet met the -ir verbs with vowel changes (e--->i) (o--->u).....

Actually, I’m guessing you are in Level II and that is the most difficult year because you get essentially ALL the grammar! So learn each tense, one at a time. How to learn best? Analyze what type of learner you are:
visual = you learn by SEEING what you want to learn, and those bilious colors of green, orange, pink, etc. can help, if you color code what won’t stick in your memory.
oral = you learn by HEARING what you want to learn. Putting things on tape with a space afterwards for repeating and later supplying the answer, is just one trick that can be used.
kinesthetic = you learn by DOING what you want to learn. This is the hardest type of learner but one that remembers far longer, just because it took more work! You can trace what you want to learn, write it on paper “footsteps” and walk on it, etc. but whatever will involve you PHYSICALLY.
Naturally, there are learners who combine 2 or even 3 of these styles of learning. Actually I need to HEAR it first (because when you SEE it first, often you ruin the pronunciation!), then SEE it and finally WRITE it.